1. Field of the Invention
This invention relates generally to valve trains for internal combustion engines and more particularly to valve trains incorporating a hydraulic tappet or lash adjuster for automatically keeping a valve clearance to zero.
2. Prior Art
As is well known in the art, a valve train for an internal combustion engine comprises intake and exhaust valves, and a valve-operating mechanism which includes a cam shaft operatively connected via a timing chain to a crankshaft for rotation about an axis thereof, and rocker arms held in engagement respectively with the intake and exhaust valves and operable by the cam shaft for pivotal movement for operating the intake and exhaust valves, respectively.
One example of such conventional valve trains comprises a pair of hydraulic tappets or lash adjusters for intake and exhaust valves, and each hydraulic tappet comprising a hollow cylinder with an open upper end and a plunger received in the cylinder for sliding movement therealong, an upper portion of the plunger extending outwardly from the cylinder. A hydraulic chamber is defined by a lower portion of the cylinder and the bottom of the plunger. A spring acts between the bottom of the cylinder and the bottom of the plunger to urge the plunger upwardly. A tubular boot made of a relatively thin film of rubber encloses the upper portion of the plunger, and the opposite ends of the rubber boot are secured fluid-tight to the upper open end of the cylinder and the upper end of the plunger, respectively, so that the rubber boot and the upper portion of the plunger cooperate with each other to define a chamber therebetween. This chamber serves as an oil reservoir and is in fluid communication with the hydraulic chamber via an oil passageway extending through the plunger and opening to the hydraulic chamber. A check valve element is provided in the hydraulic chamber and normally closes one end of the oil passageway opening to the hydraulic chamber. The upper end of the plunger of each hydraulic tappet is held against a lower surface of the mating rocker arm at one end thereof in such a manner that the rocker arm is pivotal about the upper end of the plunger. An upper end of a stem portion of each of the intake and exhaust valves is held against the lower surface of the mating rocker arm at the other end thereof. The cam shaft is held in contact with the upper surface of the rocker arm intermediate opposite ends thereof. With this construction, when any clearance tends to develop either between the rocker arm and the mating valve or between the rocker arm and the cam shaft due to thermal expansion and wear of these component parts, the plunger is moved outwardly relative to the cylinder under the influence of the spring to increase a length of the hydraulic tappet between the upper end of the plunger and the lower end of the cylinder, thereby preventing such clearance from developing. At this time, upon outward movement of the plunger to axially extend the hydraulic tappet, the check valve element is moved away from the one end of the oil passageway to cause the oil to flow into the hydraulic chamber from the oil passageway, and then the check valve element closes the one end of the oil passageway to prevent the hydraulic tappet from being axially contracted.
This conventional valve train has the following disadvantages:
(i) As described above, the opposite ends of the rocker arm are held in contact with the hydraulic tappet and the valve stem portion, and the cam shaft is held in engagement with the rocker arm intermediate opposite ends thereof. Therefore, when a clearance tends to develop either between the rocker arm and the valve stem portion or between the the rocker arm and the cam shaft, the plunger of the hydraulic tappet is moved outwardly relative to the cylinder by an amount corresponding to the clearance. Thus, the amount of axial extension of the hydraulic tappet is relatively large, which leads to an increased overall size of the engine.
(ii) The hydraulic tappet is disposed below the rocker arm and is disposed near the combustion chamber of the engine, and the boot of the hydraulic tappet is disposed in the vicinity of the combustion chamber above it. Since the boot is made of rubber, it is affected by the heat from the combustion chamber, which leads to a frequent maintenance of the hydraulic tappet.
(iii) Since the hydraulic tappet is disposed below the rocker arm, splashes of oil present in the camshaft chamber tend to impinge on the outer surface of the rubber boot. As a result, there is a possibility that such oil permeates the rubber boot and intrudes into the oil reservoir of the hydraulic tappet. This affects the function of the hydraulic tappet.
(iv) It has also been found through experiments that a small amount of ambient air tends to permeate the thin rubber boot into the oil reservoir, which also affects the function of the hydraulic tappet.